Until relatively recently, most cochlear implants were unilateral. Now, however, there are two groups of implant patients who can use the same kind of input from two ears. One group uses bilateral cochlear implants (CI). These individuals have two CIs with no residual hearing (Bilateral CI). Another group has preserved low- frequency acoustic hearing in the implanted ear and also has acoustic low-frequency hearing in the contralateral ear. These patients combine electric and acoustic stimulation (EAS) in the implanted ear and have residual low frequency hearing contralaterally. They have access to bilateral low frequency acoustic cues and are referred to as EAS. (EAS are different than bimodal CI users who have electric stimulation in one ear via a CI and acoustic hearing contralateral to the CI. In other words bimodal CI users have different hearing in each ear.) We can use these two groups of patients, Bilateral CI and EAS, with different types of bilateral hearing to assess the value of interaural level difference (ILD) cues and interaural time difference (ITD) cues in localization and in speech understanding in real world environments. Interaural time and intensity differences are known to provide normal hearing listeners with the ability to localize and are thought to underlie spatial release from masking (SRM). To date, there is a single paper (Dunn et al., 2010) on localization in EAS users. Dunn reported that EAS users are able to localize although not as well as normal hearing listeners. The study, however, lacked controls for the frequency information used in localization. There is a consensus that bilateral users have ILD sensitivity but do not have access to ITD cues (Grantham et al., 2007). Loizou et al. (2009) report bilateral CI users do not benefit from spatial release from masking, presumably due to the absence of ITD cues. If ITD cues are necessary for SRM in real world noise environments, EAS patients may benefit more in these environments than bilateral CI patients. The specific aims of this research are to determine (i) which underlying mechanism, ITD, ILD or both, underlie localization by the two groups, (ii) whether ITD or ILD cues provide a greater advantage when CI listeners are placed in difficult real world listening situations, and (iii) if there is a relationship between the availability of interaural ITD and ILD cues and SRM. The answers to these questions will shape our understanding of cochlear implant benefit and provide information to patients about real world benefits of CIs. The research design is a 2(groups=bilateral CI and EAS) X 3 (low-pass, high-pass, wideband noise) repeated measures ANOVA for localization ability from the three noise conditions and a 2 (groups = bilateral CI and EAS) X 2 (mode of listening and noise condition) repeated measures ANOVA for speech perception in noise. An analysis will be calculated to determine if performance on localization correlates with spatial release from masking. PUBLIC HEALTH RELEVANCE: The implications for a potential cochlear implant patient who is trying to decide between bilateral versus EAS could justifiably impact the decision of the patient, the surgeon, the audiologist, the tax payers, and even the insurance companies. This information will provide potential CI users and hearing health care providers with the necessary information to make an informed decision regarding expected outcomes.